A flash EEPROM memory cell (NAND or NOR), as well as many other types of memory cells known in the art, may be configured in alternate modes of operation where the cell is to either store multiple bits per cell (multi-level) or store a single bit per cell. Such a memory cell when configured to store a single bit is typically referred to as operating in a pseudo single bit cell (PSBC) mode or simply as SBC mode. While the multi-bit mode provides much higher bit density, with greater numbers of bits per cell, there is a relatively greater possibility of read errors than for the single bit per cell mode. This heightened potential for read errors arises from the small differential voltages used to store adjacent states. For example, charge coupling between memory cells of an array becomes a greater problem.
Media management techniques have been employed by multi-level cell memories to reduce the error rates while still providing high storage density, but such techniques generally come at the expense of user functionality (typically in the form of programming restrictions). The SBC mode may therefore be offered to provide enhanced user functionality at the expense of bit density so that portions of a memory array may be operated in either SBC or multi-bit mode to arrive at a combination of bit density and functionality suitable to a variety of user/host device applications.
As memory cell sizes decrease, data failure rates in the SBC mode increase. Yet, to maintain greater user functionality in SBC mode than does the multi-bit mode, the user restrictions which improved error rates in the multi-bit mode cannot merely be applied to the SBC mode. Furthermore, a reduction in error rates in the SBC mode should also not come at the expense of a greatly increased memory device size or unnecessarily increase memory device wear.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.